Hi, 

I have a 2011 Ford Fiesta 1.6 Zetec S with the M140 Mountune upgrades. 

over the last few weeks occasionally when initially pulling off it almost misfires as if there's no fuel or something similar, i have changed the spark plugs & leads but still does it occasionally.

Over the last year because of not being able to travel, it hasn't had a good run to clear the engine, would not using it so much cause a sort of mis fire? 

I think it would have to sit for quite some time to have engine issues,  modern unleaded fuel is only good for 4 to 6 months, the propane evaporates I'm told.   You've changed the obvious,  and your motor is old enough to dodge the particulate filter issue,  so i would talk to mountune in case they have knowledge of this issue with their mods,  or sadly pay to have the laptop plugged in  at a garage ?

could try a fuel cleaner like this https://www.eurocarparts.com/p/stp-petrol-treatment-200ml-549774451 ive tried the redex and found the stp better, just empty the whole bottle when you fill it up...could also be one of the fuel injectors needs changed?

 Propane is a gas so it isn't part of the fuel. That said, some of the lighter fractions evaporate, making the fuel sluggish and the ethanol soaks up water like a sponge so if the OP has had a wet weather spell and a poor tank cap seal, it is possible to dilute the fuel a bit. Try topping up with a premium fuel first and see if that fixes it. A good run never did a sound engine any harm so once it has been topped up go and put a few miles on it.